The reinforcement of thermoplastic resins by dispersing fibrous materials, such as glass fibers, so as to improve the mechanical strength properties of the resulting resin composition is well known. In particular, fiber-reinforced polyamide resins are typically produced by mixing a polyamide base resin with relatively short fibers, such as chopped fibrous strands. The polyamide base resin and short fibers are then mixed and extruded to form a fiber-reinforced polyamide resin composition. However, conventional processing techniques to obtain fiber-reinforced polyamide resin compositions cannot satisfy the need for greater mechanical strength properties, since the fiber reinforcement medium will unavoidably break into even shorter fiber segments during extrusion thereby decreasing its reinforcing effectiveness.
Investigations have been conducted recently so as to form polyamide resin compositions with a relatively long fiber-reinforcement medium in an attempt to achieve maximum reinforcement benefit from the presence of the long fibers. In this connection, it has been proposed to produce long fiber-reinforced polyamide resin compositions by simultaneously drawing the reinforcement fibers and impregnating the reinforcement fibers with a polyamide resin (for example, an emulsion, solution or melt of polyamide resin).
These prior attempts to achieve a polyamide resin composition which was reinforced with a relatively long fiber medium, however, encountered several problems. For example, such prior long fiber-reinforced polyamide resin compositions experienced molding difficulties, such as incomplete filling of the mold cavity due to hopper "breathing" (so-called "short-shots"), poor extruder screw "bite", and overall poor fluidity characteristics. Adding a conventional slip additive, such as aluminum stearate, zinc stearate or calcium stearate, to compositions which contain a long fiber reinforcement medium does not solve such problems and, in fact, creates further problems in terms of violent fuming (gas generation) when the resin is purged during molding. Furthermore, there is a tendency for the molded article to become scorched when such a material is molded, thereby detracting from its visual appearance.
The problems noted above are peculiar to polyamide resin which contain relatively long fibers as a reinforcing medium. It is presumed that these problems are attributable to the incorporation of a large amount of air into the resin during plasticization by the screw-extruder and/or by virtue of localized heat build-up caused by shear forces. However, no detailed mechanisms have been identified as to the precise causes for the problems stated above when long fibers are incorporated into a polyamide base resin. It is therefore towards providing a solution to the problems associated with incorporating a long fiber reinforcement medium into a polyamide base resin which the present invention is directed.
Broadly, the present invention is directed toward polyamide compositions which include a processing aid exhibiting selective beneficial effects when employed in combination with a relatively long fiber reinforcement medium. More specifically, the present invention is directed to polyamide resin compositions and to molded articles formed thereof which is composed of a blend of a polyamide base resin, a long fiber reinforcement medium, and a processing aid which is at least one selected from (i) a lithium salt of a fatty acid, or (ii) a metal salt of a fatty acid having between 22 to 32 carbon atoms. In preferred embodiments, the compositions of this invention will include, based on the total composition weight, between 5 to 80 wt. % of reinforcing fibers having a length of at least 3 mm, and between 0.01 to 3 wt. % of the processing aid.
The polyamide resin compositions according to the present invention are most preferably in the form of generally cylindrical pellets having a length between 3 to 50 mm. Thus, according to this invention, the fibrous reinforcing medium most preferably has a length which is substantially coextensive with, and will be oriented in the longitudinal direction of, the formed composition pellets.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.